Automobiles and other combustion engine powered vehicles typically employ an electric motor to start the combustion engine. For that purpose, the electric motor is coupled to a starting circuit which generally receives electrical power from an on-board storage battery. The starting circuit selectively couples electrical energy from the battery to the starting motor that operates to cycle the engine to initiate sustained operation. In common vehicle applications, the battery also provides electrical energy to a variety of electric power consuming devices, such as engine control electronics, lights, and vehicle accessories.
Traditional batteries for these applications, often referred to as starting, lighting and ignition (SLI) batteries, are multi-cell, lead-acid batteries. That is, the batteries are constructed from lead plates pasted with active material and arranged into stacks. Those stacks are inserted into partitioned cell compartments of a battery container, electrically interconnected, and flooded with dilute acid electrolyte. SLI batteries of this construction are more than adequate for providing the relatively high power demand required of engine starting, as well as the relatively low power demand to maintain electrical accessories during both vehicle operation and periods of non-operation. However, because of the seemingly disparate functions the SLI battery is required to perform, short duration high-power output and long duration low-power output, the battery design can not be optimized for performing either of these tasks. An additional drawback of these batteries is relatively low specific energy (kilowatt hour/gram, kWh/g) as compared to other battery constructions owing to the weight of the lead plates and the liquid electrolyte.
There has been suggested a battery system for vehicle use which includes two batteries. A first battery in the system, a starting battery, is optimized to start the engine by being specifically designed for short duration, high-power output. A second battery in the system, a reserve battery, is optimized to operate and maintain non-starting electrical loads, such as for vehicle accessories. An advantage of such a system is that the starting battery may be made smaller and lighter yet capable of providing a high power output for a short period of time. In addition, the reserve battery may be made smaller and lighter yet capable of satisfying the relatively low power requirements of vehicle accessories. In combination, the two batteries may require less space and weigh less than a single traditional SLI battery.
A limitation of a two battery system lies with maintaining the charge of both batteries. Typically, the vehicle includes a voltage/current regulation device which regulates the output of the alternator in response to the charging needs of the SLI battery and the vehicle electrical loads. In the dual battery system, each battery type delivers power and accepts charge at a different rate. For example, the starting battery delivers power at a very high rate and likewise accepts charge at a high rate. In contrast, the reserve battery delivers power at a lower rate and accepts charge at a lower rate. Moreover, it will typically be the case that each battery will be at a different state-of-charge, hence requiring different charge maintenance. Additional advantages may also be attained by selectively coupling or decoupling the batteries during inactive, starting and operational periods of the vehicle. However, careful management is required so as not to damage either the vehicle electrical system or the dual batteries.
Another problem encountered with battery powered equipment is battery drain during periods of inactivity. For example, a motor vehicle may sit parked for several weeks or months. In that situation a leakage current or current drawn by accessories left turned-on can drain the battery to a point where the remaining charge is insufficient to start the engine. Thus it is desirable to provide a control mechanism that responds to a period of inactivity by disconnecting non-essential loads from the battery.